As shown in FIG. 1, the compressor section 10 of a turbine engine is enclosed within an outer casing 12. The compressor can include a rotor 11 (partially shown) with a plurality of axially spaced discs 14. Each disc 14 can host a row of rotating airfoils, commonly referred to as blades 16. The rows of blades 16 alternate with rows of stationary airfoils or vanes 18. The vanes 18 can be mounted in the compressor section 10 in various ways. For example, one or more rows of vanes 18 can be attached to and extend radially inward from the compressor casing 12. In addition, one or more rows of vanes 18 can be hosted by a blade ring or vane carrier 20 and extend radially inward therefrom.
The compressor section 10 contains several areas in which there is a gap or clearance between the rotating and stationary components. During engine operation, fluid leakage through such clearances contributes to system losses, decreasing the operational efficiency of the engine. FIG. 2 shows one area in which fluid leakage can occur. As shown, a clearance 22 is defined between the tips 24 of the compressor vanes 18 and the substantially adjacent rotating structure, such as the rotor disc 14. Ideally, the clearance 22 is kept as small as possible, for it would result in an increase in engine performance. However, it is critical to maintain a clearance between the rotating and stationary components at all times. Rubbing of any of the rotating and stationary components can lead to substantial component/engine damage, performance degradation, and extended outages.
The size of the clearance 22 can change during engine transient operation due to differences in the thermal inertia of the rotor and discs 14 compared to the thermal inertia of the stationary structure, such as the outer casing 12 or the vane carrier 20, to which the vanes 18 are connected. The thermal inertia of the stationary structure (outer casing 12 and/or the vane carrier 20) is significantly less than the rotating structure (rotor and/or the discs 14). Thus, the stationary structure has a faster thermal response time and responds (through expansion or contraction) more quickly to a change in temperature than the rotating structure. These differences in thermal inertia give rise to the potential for vane tip rubbing.
Prior efforts have sought to avoid vane tip rubbing. To that end, large tip clearances 22 are initially provided so that the vane tips 24 do not rub during non-standard engine conditions where the clearances 22 would otherwise be expected to be the smallest. Examples of such non-standard operating conditions include hot restart (such as, restarting the engine soon after shutdown, spin cool, etc.). However, because the minimum tip clearances 22 are sized for these off design conditions, the clearances 22 become overly large during normal engine operation, such as at base load. Consequently, the compressor and the engine overall can experience measurable performance decreases in power and efficiency due to tip clearance leakage.
Thus, there is a need for a system that can improve engine performance by minimizing vane tip clearances.